Conventionally, an optical communication system in which a transmitter transmits an optical signal and a receiver receives the optical signal through an optical fiber serving as a transmission path has been used. In such an optical communication system, an optical amplifier is used to compensate for a loss of a transmission path.
The optical amplifier used in the optical communication system includes a post amplifier arranged on an output side of the transmitter, a preamplifier arranged in front of the receiver, or an inline amplifier used for multistage relay. In recent years, an EDFA (Erbium-Doped Fiber Amplifier) using an erbium-doped fiber has been broadly used for an optical communication system of a 1550-nm band.
Furthermore, as a demand for communication has increased owing to wide use of the Internet, a wavelength division multiplexing (WDM) system which makes use of a broadband characteristic of an optical amplifier has been developed. In recent years, a ROADM (Reconfigurable Optical Add/Drop Multiplexer) system in which the WDM system is combined with a wavelength routing technique as shown in FIG. 27 has been employed in a metropolitan ring network in order to flexibly and efficiently operate an optical transmission network. The ROADM system includes optical components such as an optical branching coupler, an optical switch, and a wavelength multiplexer/demultiplexer. The optical amplifier is used to compensate for losses in the optical components.
In such a ROADM system, as light amplified by the optical amplifier, an amplified optical signal and an amplified spontaneous emission (ASE) are output. Since the ASE is generated in a random manner and interferes with the signal light beam, the ASE corresponds to a noise component for the signal light beam in the optical communication system. That is, a performance of an optical receiver in the ROADM system is also limited by this noise component. Therefore, in the optical communication system, it is important to measure an optical signal component and a noise component. Normally, as an index of a capability of the optical receiver, an OSNR (Optical Signal-to-Noise Ratio) resistance calculated as a ratio of an optical signal component to a noise component is used.
Furthermore, in a broadband access network, as a FTTH (Fiber To The Home) is broadly used, demands for audio data, video data which requires a high capacity, and the like have been increased. Furthermore, a transmission rate per wavelength in the WDM system has been shifted from 10 Gbps to 40 Gbps. In an optical communication system having a transmission rate of 40 Gbps or larger, a pulse interval of one bit width is smaller and a frequency band of a signal is larger when compared with an optical communication system having a transmission rate of 10 Gbps. Therefore, the optical communication system having the transmission rate of 40 Gbps or larger is affected by noise of an optical amplifier, wavelength dispersion of a transmission-path optical fiber, and polarization mode dispersion, and accordingly, a transmission characteristic is considerably deteriorated.
Accordingly, as a method different from relevant OOK (On Off Keying), an optical modulation method such as DPSK (Differential Phase Shift Keying) or DQPSK (Differential Quadrature Phase Shift Keying) is employed, for example. Note that an example of a type of the DPSK modulation method includes NRZ-DPSK (Non Return to Zero-DPSK).
Here, a method for calculating an OSNR when the various phase modulation methods described above are employed in the ROADM system will be described. First, a calculation equation for the OSNR will be described. The calculation equation for the OSNR is represented by a ratio of an optical signal output power (Psig [mW]) relative to an ASE power (Pase, 0.1 nm [mW]) in a signal wavelength of a 0.1-nm band.
                    Expression        ⁢                                  ⁢                  (          1          )                                                                              OSNR          ⁡                      [            dB            ]                          =                  10          ×                      log            ⁡                          (                              Psig                                  Pase                  ,                                      0.1                    ⁢                                                                                  ⁢                    nm                                                              )                                                          (        1        )            
Next, an optical signal output power (Psig [mW]) and an ASE power (Pase, 0.1 nm [mW]) included in Expression (1) will be described in detail. A WDM EDFA is employed as a preamplifier of ROADM nodes shown in FIG. 27, and a spectrum output from the WDM EDFA, i.e., a spectrum supplied to a demultiplexer after being amplified is shown in FIG. 28. Note that FIG. 28 is a graph illustrating an example of a spectrum supplied to the demultiplexer after being amplified in an optical communication system according to the related art.
For example, as shown in FIG. 28, the WDM EDFA amplifies a signal of a 1550-nm band and generates a wide-band ASE for approximately 40 nm. In a ROADM system which is a long-distance transmission optical system including multistage-relay WDM EDFAs, the ASE is increased every time the ASE is supplied through the WDM EDFAs.
On the other hand, each of receivers in the ROADM system receives a spectrum of one of waves obtained by demultiplexing a spectrum output from a WDM EDFA using the wavelength demultiplexer, that is, each of the receivers receives a spectrum in which an optical signal and an ASE are superposed with each other as shown in FIG. 29. Note that FIG. 29 is a diagram illustrating a spectrum obtained through demultiplexing performed by the wavelength demultiplexer in an optical communication system according to the related art.
Accordingly, when an OSNR is to be obtained, an optical spectrum as shown in FIG. 29 received by a receiver is measured using an optical spectrum analyzer, an optical signal power and an ASE power are separated from the measured optical spectrum, and the obtained optical power values are assigned to Expression (1) to be calculated.    Related Art Document: Japanese Unexamined Patent Application Publication No. 2008-098975
However, in the related art described above, there arises a problem in that an optical spectrum analyzer having a complicated optical configuration is required. Furthermore, since a signal spectrum spreads at a time of signal modulation, it is difficult to separate the optical signal power and the ASE power from the optical spectrum, and therefore, reliable monitoring is not ensured.
For example, as a signal spectrum obtained when a signal modulation is performed on condition that the DQPSK is employed, a symbol time is equal to 1/21.5 GHz, an optical fiber bandwidth f-3 dB is equal to 0.26 nm, and an OSNR is equal to 20 dB, (1) a signal spectrum 111 which is obtained upon modulation and which represents an ASE spectrum and (2) a signal spectrum 121 obtained after being supplied through a demultiplexer of a bandwidth of 0.26 nm are shown in FIG. 30. As for the signal received (DROP) by a receiver or a signal transmitted (ADD) by a transmitter, bands of both ends in a horizontal direction are cut as denoted by (2) of FIG. 30 by an optical filter, a demultiplexer, or the like, and therefore, determination as to whether this signal corresponds to an ASE is not reliably made. That is, since it is difficult to separate an optical signal power and an ASE power from a signal spectrum at a time of signal modulation, reliable monitoring is not performed. Note that FIG. 30 is a diagram illustrating a signal spectrum at a time of signal modulation in an optical communication system according to the related art.
Furthermore, a method for measuring a BER (Bit Error Rate) may be employed as a method for monitoring an optical transmission quality. However, in this method, since deterioration of wavelength dispersion, PMD (Polarization-Mode Dispersion), XPM (Cross Phase Modulation), FWM (Four-Wave Mixing), and the like are measured in addition to deterioration of the OSNR, the OSNR is not accurately monitored.